Method for producing non-colored polyacrylate adhesive compounds with a narrow molar mass distribution

ABSTRACT

Method for producing polyacrylate adhesive compounds with a narrow molar mass distribution by radical polymerization and subsequent decolorization of colored reaction products resulting therefrom.

This is a 371 of PCT/EP2012/069288 filed 28 Sep. 2012, which claimsforeign priority benefit under 35 U.S.C. §119 of German PatentApplication No. 10 2011 086 502.0 filed Nov. 16, 2011, the entirecontents of which are incorported herein by reference.

The present invention relates to a method for producing uncoloredpolyacrylate adhesives having narrow molar mass distribution by means offree-radical polymerization, and to products which are produced withthese adhesives.

BACKGROUND OF THE INVENTION

Industrial adhesive tape applications very often use pressure sensitivepolyacrylate adhesives. Polyacrylates possess diverse advantages overother elastomers. They are highly stable toward UV light, oxygen, andozone. Synthetic and natural rubber adhesives usually contain doublebonds, which render these adhesives unstable toward the aforementionedenvironmental influences. Another advantage of polyacrylates is theirtransparency and their capacity for use across a relatively broadtemperature range.

Pressure sensitive polyacrylate adhesives are prepared generally insolution by a free radical polymerization. Generally speaking, thepolyacrylates in solution are coated via a coating bar onto the carriermaterial in question, and are subsequently dried. In order to raise thecohesion, the polymer is generally crosslinked. Curing proceedsthermally, by UV crosslinking (ultraviolet radiation), or by EB curing(EB: electron beams). The operation described is relatively costly andinconvenient and is environmentally objectionable, since the solvent isgenerally not recycled, and a high level of organic solvent consumptionentails a high environmental burden.

It is very difficult, moreover, to produce pressure sensitive adhesivetapes with high coatweight without bubbles.

An improvement in these disadvantages is provided by the hotmeltprocess. In this hotmelt process, the pressure sensitive adhesive (PSA)is applied in the melt to the carrier material.

This new technology, however, also entails restrictions. Before thecoating operation, the solvent is removed from the PSA in a dryingextruder. The drying procedure involves a relatively high temperatureand shearing exposure, causing high damage particularly to highmolecular mass polyacrylate PSAs. The acrylate PSA gels, or the lowmolecular mass fraction is increased greatly as a result of molecularweight reduction. Both effects are undesirable, being deleterious touse. Either the adhesive can no longer be coated, or else there arechanges in the technical properties of the PSA, since, for example, onexposure of the adhesive to a shearing force, the low molecular massfractions act as lubricants and so lead to premature failure of theadhesive.

A solution to avoiding these disadvantages is offered by polyacrylateadhesives with a low average molecular weight and narrow molecularweight distribution. Here, the polymerization procedure greatly reducesthe proportion of low molecular mass and high molecular mass moleculesin the polymer. The removal of the high molecular mass fractions lowersthe flow viscosity, and the adhesive exhibits less of a tendency to gel.The lowering in the low molecular mass fraction reduces the number ofoligomers that lower the shear strength of the PSA.

A variety of polymerization techniques are suitable for the productionof low molecular mass PSAs. State of the art is the use of chaintransfer agents (CTAs), such as of alcohols or thiols, for example.These CTAs reduce the molecular weight, but broaden the molecular weightdistribution.

Another control polymerization technique employed is that of AtomTransfer Radical Polymerization, ATRP, where preferably monofunctionalor difunctional secondary or tertiary halides are used as initiator, andcertain metal complexes are used for the purpose of abstracting thehalide or halides. As a side-effect, however, the metal catalystsemployed generally influence the aging of the PSAs in a negative way(gelling, transesterification). Moreover, the majority of metalcatalysts are toxic, discolor the adhesive, and are removable from thepolymer only by costly and inconvenient precipitations.

Other controlled radical polymerization processes utilize a compound ofthe formula R′R″N—O—X, for example, as initiator, in which X representsa free radical species which is able to polymerize unsaturated monomers;very specific radical compounds, such as phosphorus-containingnitroxides or specific nitroxyls, for example, which are based onimidazolidine, morpholines, piperazinones, or piperazinediones. Ingeneral, however, the reactions exhibit low conversion rates,particularly for the polymerization of acrylates, leading to very lowyields and molecular weights, and/or relatively high temperatures areneeded in order to shift the equilibrium between the “dormant” and theactive radical species to the side of the active species, and in order,thus, to increase the net reaction rate. The choice of solvents istherefore confined essentially to high-boiling solvents.

One suitable method for producing narrow-range polymers is that known asthe RAFT procedure (Reversible Addition-Fragmentation Chain Transfer),described for example in specifications WO 98/01478 A1 and WO 99/31144A1. The procedure described therein is not immediately suited to theproduction of PSAs, since the conversions achieved are very low and theaverage molecular weight of the polymers prepared is too low for PSAs,especially those based on acrylate. Consequently, the polymers producedin this way cannot be employed as acrylate PSAs.

Onward developments of this procedure, through the introduction ofthioesters or trithiocarbonates, are a topic of research. For instance,EP 1 626 994 A1 describes improved RAFT CTAs, with which it has provedpossible to produce polyacrylate PSAs for hotmelt coatings. The RAFTprocedure has significant advantages over ATRP and overnitroxyl-controlled polymerization, since there is no need to accept areduction in reaction rate, nor to use expensive and in some casesunstable catalysts, and the RAFT CTAs are more universal in theirusefulness.

Generally speaking, nevertheless, acrylate PSAs obtained by means ofsulfur-containing RAFT reagents of these kinds have disadvantages fornumerous spheres of use. Polymers prepared with RAFT CTAs, especially(pressure sensitive) adhesives, do in fact have very defined polymerconstructions and polydispersities, and can therefore be adjusted veryeffectively in terms of their technical adhesive properties. By thenature of the RAFT CTAs, however, because of the chromophoric sites ofthe functional groups (having conjugated double bonds containingespecially sulfur atoms, optionally oxygen or nitrogen atoms), thepolymers obtained are generally relatively intense yellow to brown incolor, which is intolerable for a multitude of applications in theadhesive tapes sector. Moreover, production-related sulfur fragments inthe polymers, by-products and CTA residues can have a very unpleasantodor and are typically themselves colored.

It is therefore an object of the invention to provide a correspondingpolymerization method, by which polyacrylate adhesives having a narrowmolar mass distribution can be prepared, but which has the disadvantagesof the prior art cited only to a reduced degree, if at all. Significantdisadvantages include the slow reaction rate of the ATRP and thenitroxyl-controlled polymerization, and also the odor and the color ofthe RAFT CTAs.

It has been found that, surprisingly, the combination of a RAFT processwith subsequent addition of a diene compound after completion of thepolymerization afforded polyacrylate pressure-sensitive adhesive systemsthat are virtually colorless and odorless, but were neverthelessprepared utilizing the advantages of the RAFT process, namely the highreaction rate and the narrow molar mass distribution.

SUMMARY OF THE INVENTION

The invention therefore relates to a method for preparing acrylate-basedpolymers, wherein first a controlled radical polymerization reaction ofan initial reaction charge comprising at least one acrylate-basedmonomer, in other words of an acrylate-based monomer or of a monomermixture comprising at least one acrylate-based monomer, in each casecustomarily in the presence of solvents, is carried out in the presenceof at least one chain transfer agent (CTA) having a functional groupS—C═X, where X═S, O or N, with the CTA being selected from the groupencompassing dithioesters, i.e., compounds of the general structure

dithiocarbonates, and specifically both the S,S′-substituteddithiocarbonates, i.e., compounds of the general structure

and xanthates (O,S-substituted dithiocarbonates), i.e., compounds of thegeneral structure

dithiocarbamates, i.e., compounds of the general structure

trithiocarbonates, i.e., compounds of the general structure

andimidodithiocarbonates, i.e., compounds of the general structure

where, above, R generally and independently at each occurrenceselectedly represents organic radicals or else, optionally, inorganicradicals (the members of the stated group of CTAs are also referred toin the context of this specification as “RAFT CTAs” or as polymerizationregulators).

The majority of the CTA molecules, and more particularly virtually allCTA molecules, are incorporated into the polymer chains that form insuch a way that the respective polymer chain has at least one functionalgroup of a CTA incorporated into it. In accordance with theinvention—preferably after completion of the polymerization reaction—atleast one chemical compound which has at least two conjugated doublebonds (this chemical compound is referred to hereinafter as “conjugateddiene”) is brought into contact with the polymers thus obtained, suchthat hetero-Diels-Alder reactions are brought about between the doublebonds C═X (with X═S, O or N) of the functional groups S—C═X incorporatedinto the polymer chains and the conjugated double bonds of theconjugated diene, as a result of which the chromophoric sites inparticular react and decolorization takes place.

DETAILED DESCRIPTION

Where reference is made in the context of this specification to a“chemical compound” or a “chemical substance”—such as, for example, evena monomer, a CTA, an initiator, or the like—the reference is not to theindividual molecule, but instead to the nature of this chemical compoundor substance, i.e., the respective group of identical molecules. Wherethe individual molecule is meant, the reference will be to the moleculeof the corresponding chemical substance (in other words, for example, toa monomer molecule, a CTA molecule, or an initiator molecule,respectively). The expression “two or more chemical compounds” (forexample, among other things, two or more monomers, two or more CTAs, andso on) accordingly denotes two or more groups of identical molecules(for example, “two or more monomers” means two or more groups ofidentical monomer molecules in the respective group, with the monomermolecules differing between the respective groups; “two or more CTAs”means two or more groups of identical CTA molecules in the respectivegroup, with the CTA molecules differing between the respective groups;and so on). Where, in contrast, a functional group is referred to, thereference is to the individual functional group, unless otherwiseindicated in the specific case.

The method of the invention permits the preparation of well-definedacrylate polymers, which can be outstandingly employed as adhesives,more particularly as PSAs, or can be processed further to suchadhesives/PSAs.

The term, “pressure sensitive adhesive” (PSA) refers, as is customary,to those visoelastic, polymeric compositions which—optionally as aresult of appropriate additization with further components, such astackifier resins, for example—are durably tacky and permanently adhesiveat the application temperature (room temperature unless otherwisedefined) and adhere to a multiplicity of surfaces on contact, withadhesion more particularly being instantaneous (which exhibit what iscalled “tack” [also referred to as stickiness or touch-stickiness]).They are capable, even at the application temperature and withoutactivation by solvent or by heat, but optionally under the influence ofa more or less high pressure, of wetting a bonding substratesufficiently to allow interactions sufficient for adhesion to developbetween the composition and the substrate.

The polymerization is initiated advantageously by one or more radicalinitiators. Suitable radical initiators for the polymerization include,in particular, thermally decomposing initiators, especiallyradical-forming azo or peroxo initiators. The initiator or initiatorsare preferably added before and/or in the course of the polymerization.Multiple initiation is preferred, for which a first addition ofinitiator is made before or at the beginning of the polymerization, andfor which at least one further addition of initiator takes place in thecourse of the polymerization; advantageously, the addition of furtherinitiators is made in at least two method stages. In this case, in eachstep of addition, it is possible to use the initiator employed first, aninitiator already employed before, or a different initiator. Suitable inprinciple are all customary initiators known for acrylates.

Examples of radical sources are peroxides, hydroperoxides, and azocompounds; a number of nonexclusive examples of typical radicalinitiators include potassium peroxodisulfate, dibenzoyl peroxide, cumenehydroperoxide, cyclohexanone peroxide, di-tert-butyl peroxide,azo-bis(isobutyronitrile), cyclohexylsulfonyl acetyl peroxide,diisopropyl percarbonate, bis(4-tert-butylcyclohexyl) peroxydicarbonate(Perkadox® 16 from Akzo Nobel), tert-butyl peroctoate, and benzopinacol.In one very preferred version, radical initiators used are2,2′-azo-bis-(2-methylbutyronitrile) (Vazo 67® from DuPont) and/or1,1′-azo-bis-(cyclohexanecarbonitrile) (Vazo 88® from DuPont).

It is also possible, furthermore, to use radical sources which releaseradicals only on irradiation with UV light.

For the thermally decomposing initiators, the introduction of heat isessential to the initiation of the polymerization. The polymerizationfor the thermally decomposing initiators can be initiated moreparticularly by heating to 50 to 160° C., depending on initiator type.For the use of UV initiators, UV light of the appropriate wavelength isbeamed in. This reaction may be carried out more particularly in atemperature range from 0° C. to 150° C.

In one advantageous development of the method, thepolymerization—especially via presence of at least one radicalinitiator—is carried out with at least one dithioester and/ortrithiocarbonate as polymerization regulator. In one preferred variantof the inventive method, RAFT CTAs used are compounds of the followinggeneral structural formula

where Q and R¹ are selected independently of one another, and Q ispreferably a radical from one of groups a) to n), and R¹ is preferably aradical from one of groups a), c) to f), or h):

-   a) branched and unbranched C₁ to C₁₈ alkyl, branched and unbranched    C₃ to C₁₈ alkenyl, and branched and unbranched C₃ to C₁₈ alkynyl    radicals-   b) ethenyl and ethynyl radicals-   c) unfused and fused aryl radicals, especially C₆ to C₁₈ aryl    radicals, more particularly unsubstituted or substituted phenyl    radicals, and unsubstituted or substituted benzyl radicals-   d) aliphatic heterocyclic radicals, more particularly C₃ to C₁₂    cycloalkyl radicals-   e) aromatic heterocyclic radicals-   f) substituted radicals of group a), more particularly    -   f1) radicals of group a) each substituted by at least one OH        group, halogen atom or silyl ether,    -   f2) radicals of group a) each substituted by at least one ester,        amine, carbonate, cyano, isocyano and/or epoxide group and/or by        sulfur,-   g) substituted radicals of group a), more particularly    -   g1) radicals of group b) each substituted by at least one OH        group, halogen atom or silyl ether,    -   g2) radicals of group b) each substituted by at least one ester,        amine, carbonate, cyano, isocyano and/or epoxide group and/or by        sulfur,-   h) —NH₂, —NHR^(I), —NR^(I)R^(II), —NH—C(O)—R^(I), —NR¹—C(O)—R^(II),    —NH—C(S)—R^(I), —NR¹—C(S)—R^(II),

-   -   where R^(I) and R^(II) are radicals selected independently of        one another from groups a) to g)

-   i) —S—R^(I), —S—C(S)—R^(I), where R^(I) is a radical selected from    one of groups a) to g),

-   k) —O—R^(I), —O—C(O)—R^(I), where R^(I) is a radical selected from    one of groups a) to g),

-   l) radicals containing phosphate groups, more particularly    —P(O)(OR^(III))(OR^(IV)), where R^(III) and R^(IV) are identical or    radicals selected independently of one another from groups a) to g),

-   m) C₂ to C₁₈ heteroalkyl radicals having at least one O atom and/or    at least one NR^(I) group in the carbon chain, where R^(I) is a    radical selected from one of groups a) to g),

-   n) hydrogen.

The stated substituent listings serve only as examples of the respectivegroups of compounds, and make no claim to completeness.

Also suitable as polymerization regulators are compounds of thefollowing types

where R², R³, and R⁴ are selected independently of one another fromgroups a) to m).

In a particularly preferred procedure in accordance with the invention,the following compounds

are used as polymerization regulators, where R*, R** and R***, ingeneral terms and each selected independently, are organic radicals orelse optionally inorganic radicals; more particularly, these regulatorsare used in the form of the compounds

where R², R^(III) and R^(IV) are defined as above.

With great advantage, the quantity of the RAFT CTAs is selected suchthat they are employed in total with an (overall) weight fraction of0.001%-5%, more particularly of 0.025% to 0.25%, based on the monomers.In the inventive sense, moreover, it is very useful if the molar ratioof radical initiator added first (first step of addition) to the amountof all RAFT CTAs is in the range from 50:1 to 1:1, more particularlybetween 10:1 and 2:1.

The method is particularly outstandingly suitable for the preparation ofacrylate-based polymers, these being those polymers which in part, moreparticularly predominantly (i.e., to an extent of more than 50 wt %),are attributable to acrylic acid, methacrylic acid, acrylic estersand/or methacrylic esters as monomers (referred to hereinaftercollectively as “acrylic monomers”) (where reference is made generally,for the purposes of this specification, to “acrylic” or “acrylate”, theintention is to include therein the corresponding methyl-substitutedderivatives, in other words the methacrylic compounds, unlessspecifically maintained otherwise). Equally, the expression“(meth)acrylic” embraces the corresponding acrylic compounds and thecorresponding methacrylic compounds.

The method is particularly suitable for the preparation of acrylatePSAs. PSAs consist customarily of a polymer component, also referred toas base polymer component, which may be a homopolymer, a copolymer, or ablend of polymers (homopolymers and/or copolymers). The composition ofthe polymer component may be selected according to the desiredproperties of the PSA. The base polymer component is customarily admixedwith further additions, sometimes to a considerable extent, in order toobtain the desired properties for the end product (the PSA). PSAs arefrequently crosslinked, in order to bring about sufficient cohesion.

The starting point advantageously in accordance with the invention is aninitial reaction charge, more particularly a monomer mixture, in whichthere are ethylenically unsaturated compounds, comprising moreparticularly (meth)acrylic acid and/or derivatives thereof, and thisinitial reaction charge is polymerized radically via a RAFT process,using RAFT CTAs.

The polymer component of the polyacrylate PSA advantageously comprisesone or more polyacrylates obtainable in each case by (co)polymerizationof

-   (a1) 70 to 100 wt % of acrylic esters and/or methacrylic esters    and/or the corresponding free acids, with the formula

-   -   where R⁵═H and/or CH₃, and R⁶═H and/or alkyl chains having 1 to        30 C atoms, with

-   (a2) 0 to 30 wt % of olefinically unsaturated monomers with    functional groups.

The weight figures are based on the respective polyacrylate.

This polyacrylate or these polyacrylates may constitute the base polymercomponent of the pressure-sensitive adhesive, or else may be blendedwith other polymers (acrylate polymers or other polymers) to give thebase polymer component.

The polymer component is especially advantageously selected such thatthe total content of acrylate monomers is 40 to 100% by weight, morepreferably 50 to 100% by weight.

Used preferably for the monomers (a1) are acrylic and/or methacrylicesters with alkyl groups having 1 to C atoms. These acrylic monomers maybe selected exemplarily and advantageously from the following list,encompassing methyl acrylate, methyl methacrylate, ethyl acrylate, ethylmethacrylate, propyl acrylate, propyl methacrylate, n-butyl acrylate,n-butyl methacrylate, n-pentyl acrylate, n-hexyl acrylate, n-hexylmethacrylate, n-heptyl acrylate, n-octyl acrylate, n-nonyl acrylate,lauryl acrylate, stearyl acrylate, stearyl methacrylate, behenylacrylate, and the corresponding branched isomers, such as 2-ethylhexylacrylate, for example. Other classes of compound for use, which maylikewise be added in small amounts under (a1), are cyclohexylmethacrylates, isobornyl acrylate, and isobornyl methacrylates.

Used exemplarily and preferably for (a2) are monomers of the followinglist, encompassing maleic anhydride, itaconic anhydride, glycidylmethacrylate, benzyl acrylate, benzyl methacrylate, phenyl acrylate,phenyl methacrylate, tert-butylphenyl acrylate, tert-butyl-phenylmethacrylate, phenoxyethyl acrylate, phenoxyethyl methacrylate,2-butoxyethyl methacrylate, 2-butoxyethyl acrylate, dimethylaminoethylmethacrylate, dimethylaminoethyl acrylate, diethylaminoethylmethacrylate, diethylaminoethyl acrylate, and tetrahydrofurfurylacrylate, hydroxyethyl acrylate, 3-hydroxypropyl acrylate, hydroxyethylmethacrylate, 3-hydroxypropyl methacrylate, 4-hydroxybutyl acrylate,4-hydroxybutyl methacrylate, allyl alcohol, itaconic acid, acrylamide,and cyanoethyl methacrylate, cyanoethyl acrylate, 6-hydroxyhexylmethacrylate, N-tert-butylacrylamide, N-methylolmethacrylamide,N-(butoxy-methyl)methacrylamide, N-methylolacrylamide,N-(ethoxy-methyl)acrylamide, N-isopropylacrylamide, vinylacetic acid,β-acryloyloxypropionic acid, trichloroacrylic acid, fumaric acid,crotonic acid, aconitic acid, dimethylacrylic acid, 4-vinylbenzoic acid,this enumeration not being conclusive.

Also used preferably for the component (a2) are aromatic vinylcompounds, in which the aromatic nuclei are based preferably on C₄ toC₁₈ units, but may also contain heteroatoms. Particularly preferredexamples are styrene, 4-vinylpyridine, n-vinylphthalimide,methylstyrene, and 3,4-dimethoxystyrene, this enumeration not beingconclusive.

For the polymerization, the monomers are selected such that theresultant polymers can be employed as thermally crosslinkable PSAs, moreparticularly such that the resulting polymers possesspressure-sensitively adhesive properties in line with the “Handbook ofPressure Sensitive Adhesive Technology” by Donatas Satas (van Nostrand,New York 1989).

The nature of the comonomers is selected such that the glass transitiontemperature T_(g,A) of the polymers (glass transition temperatures areunderstood for the purposes of this specification to be the static glasstransition temperatures as determinable via Differential Scanningcalorimetry (DSC) in accordance with DIN 53765; the figures for theglass transition temperature T_(g) in the context of this specificationrelate to the glass transformation temperature value Tg in accordancewith DIN 53765:1994-03, unless specifically indicated otherwise) isbelow the application temperature, preferably at T_(g,A)<=15° C. Inorder to achieve this, furthermore, the quantitative composition of themonomer mixture is advantageously selected such that in accordance withthe Fox equation (E1) (cf. T. G. Fox, Bull. Am. Phys. Soc. 1956, 1,123), the desired T_(g,A) value for the polymer is obtained.

$\begin{matrix}{\frac{1}{T_{g}} = {\sum\limits_{n}\; \frac{W_{n}}{T_{g,n}}}} & \left( {E\; 1} \right)\end{matrix}$

In this equation, n represents the serial number of the monomers used,W_(n) the mass fraction of the respective monomer n (wt %), and T_(g),the respective glass transition temperature of the homopolymer of therespective monomer n in K.

The polymers prepared preferably have a weight-average molecular weightM_(w) of 50 000 to 600 000 g/mol, more preferably between 100 000 and500 000 g/mol (the determination of average molecular weights M_(w) andM_(n) and of polydispersities P is carried out by means of sizeextrusion chromatography [gel permeation chromatography, GPC];calibration: PMMA standards [polymethyl methacrylate calibration]).Depending on reaction regime, the acrylate PSAs produced by this methodpossess a polydispersity P of M_(w)/M_(n)<4.5. The inventive method ismore preferably carried out such that the molecular weight distributionof the polyacrylates features a polydispersity of 2 to 3.5.

The polymerization may be carried out in bulk, in the presence of one ormore organic solvents, in the presence of water, or in mixtures oforganic solvents and water. The aim is to minimize the amount of solventused. Examples of suitable organic solvents are pure alkanes (e.g.,hexane, heptane, octane, isooctane), aromatic hydrocarbons (e.g.,benzene, toluene, xylene), esters (e.g., ethyl acetate, propyl, butyl,or hexyl acetates), halogenated hydrocarbons (e.g., chloro-benzene),alkanols (e.g., methanol, ethanol, ethylene glycol, ethylene glycolmonomethyl ether), ketones (e.g., acetone, butanone), and ethers (e.g.,diethyl ethers, dibutyl ethers), or mixtures thereof. The aqueousreaction systems for aqueous polymerization reactions may be admixedwith a water-miscible or hydrophilic cosolvent, in order to ensure thatthe reaction mixture is present in the form of a homogeneous phaseduring monomer conversion. Cosolvents for the present invention areselected preferably from the following group, consisting of aliphaticalcohols, glycols, ethers, glycol ethers, pyrrolidines,N-alkyl-pyrrolidinones, N-alkylpyrrolidones, polyethylene glycols,polypropylene glycols, amides, carboxylic acids and salts thereof,esters, organic sulfides, sulfoxides, sulfones, alcohol derivatives,hydroxyether derivatives, aminoalcohols, ketones, and the like, and alsoderivatives and mixtures thereof.

Depending on conversion and temperature, the polymerization time iscustomarily between 4 and 72 hours. The higher the reaction temperaturethat can be selected, in other words the higher the thermal stability ofthe reaction mixture, the lower the reaction time that can be selected.

Also included in the subject matter of the invention is the reaction ofthe functional groups in the polymerization product that come from theRAFT CTAs with conjugated dienes subject to hetero-Diels-Aldermechanisms. This gives rise to a method for decolorization and/ordeodorization of the polymers, especially for use thereof as anadhesive, more preferably as a polyacrylate pressure-sensitive adhesive.

Surprisingly it has been found that through the addition of conjugateddienes, the discoloration, caused by the RAFT reagents and/or byproductsand elimination products that arise during the preparation or reactionof the RAFT reagents, is significantly minimized and in some casesremoved completely, without change to the adhesive properties of thecomposition. The odor of the sulfur compounds as well is in partreduced, and so is no longer perceived as a nuisance, or is notperceived at all, in the product.

The conjugated diene or dienes here may be added (admixed)advantageously to the resulting polymers, more particularly at a time atwhich the polymerization is largely or already completely at an end.This therefore prevents the CTAs losing their activity as a result ofhetero-Diels-Alder reactions, where this activity is still necessary forthe polymerization.

Suitable in accordance with the invention are all diene components ableto enter into a hetero-Diels-Alder reaction with the double bonds C═X ofthe functional groups S—C═X from the RAFT reagents, more particularly inthe polymer chains, as dienophile, as is shown schematically below:

Also contributing to the decolorizing are Diels-Alder reactions whichproceed with the corresponding groups C═X in byproducts and superfluousCTA residues from the polymerization.

The activity of the conjugated dienes in terms of the functional groupsoriginating from the CTAs can be determined by the skilled person bymeans of familiar considerations, without undue effort, as for instanceby means of the Woodward-Hoffmann rules or using HOMO-LUMO approaches,and also, in the case of cyclic dienes, by approaches involving thereduction of ring strain. These theories are described comprehensivelyin numerous textbooks, and belong to the knowledge bank of the skilledperson.

The dienes are very preferably selected such that they are also capableof binding thiols—which occur frequently as byproducts and usually areresponsible for, or at least contribute to, the odor—by means ofhydrothiolation and/or thiol-ene reaction.

The dienes used are preferably acyclic and cyclic compounds having twoconjugated double bonds, more preferably electron-deficient acyclic orcyclic compounds having two conjugated double bonds. Examples of thedienes of the invention are 1,3-butadiene, 1-methoxy-1,3-butadiene,2,4-hexadiene, (2E)-2,4-pentenedienoic acid, sorbic acid and its esters,2,4-hexadiene-1,6-dicarboxylic acid and its esters, 1,3-cyclohexadiene,1-methoxy-1,3-cyclohexadiene, anthracene, substituted and unsubstitutedfurans, substituted and unsubstituted thiophenes. Particularly preferredare substituted and unsubstituted cyclopentadienes,(E)-1-methoxy-3-trimethylsilyloxy-1,3-butadiene (Danishefsky diene),1-(trimethylsiloxy)-1,3-butadiene, 1-(triethylsiloxy)-1,3-butadiene, and(1E,3Z)-1-methoxy-2-methyl-3-(trimethylsilyloxy)-1,3-pentadiene asdienes since these already lead to decolorization without catalyst atroom temperature within a few minutes.

In addition to or as an alternative to added conjugated dienes, thedienes can also be produced in situ, especially by addition of one ormore of those chemical compounds from which conjugated dienes can beproduced in situ to the polymers obtained.

In addition, in accordance with the invention, the decolorization canthus also be brought about by means of those dienes that are capable ofDiels-Alder reaction and are obtainable in situ, such aso-quinodimethane, for example, which may be prepared, among other ways,by means of metal catalysts from 1,2-bis(chloromethyl)benzene or1,2-bis(bromomethyl)benzene, and also photochemically by means of UVlight from (2,5-dimethylphenyl)(phenyl)-methanone derivatives.

Preference is given to the use of those conjugated dienes (admixedand/or generated in situ) for which the conjugated double bonds in theunit C═C—C═C are not substituted by heteroatoms.

Having proved to be very preferred in the sense of the invention is thecombination of a dithioester as RAFT reagent, more particularly adithioester according to one of the formulae IV and/or V above, andcyclopentadiene as conjugated diene for decolorization of the polymers,especially of the (pressure-sensitive) adhesive.

The method for decolorization of the polymers, especially of the(pressure-sensitive) adhesive, preferably takes place in solution, butcan also be conducted in the melt or in dispersions, in which case thereaction can proceed either thermally or photochemically. Thedecolorization reaction preferably takes place at temperatures up to120° C.; more preferably, it takes place under mild conditions up to 40°C. The reaction at low temperatures can advantageously be catalyzed byaddition of a Lewis acid (e.g. ZnCl₂) and/or of a Brønsted acid(trifluoroacetic acid, p-toluenesulfonic acid).

The invention additionally provides the polymers, more particularlyadhesives, preferably pressure sensitive adhesives, that are obtained bythe method of the invention—including the decolorizing step.

Through the decolorizing reaction, the polymers obtained, moreparticularly the adhesives or PSAs, have cylco-1-X-hex-3-ene units inthe polymer chains, and the hex-3-ene subunit may optionally also besubstituted by heteroatoms (depending on the conjugated dienes used).Further provided by the invention, therefore, are acrylate-basedpolymers, more particularly in the form of adhesive, preferably pressuresensitive adhesive, which comprise cyclo-1-X-hex-3-ene units with X═S, Oor N in the polymer chains, more particularly those polymers of thiskind that are obtainable by the method of the invention. The majority ofthese polymers will have exactly one such cyclo-1-X-hex-3-ene unit perpolymer chain.

According to adhesive or self-adhesive properties already present, thepolymers of the invention, as already indicated, may be used veryeffectively as adhesives, more particularly as PSAs, or may befurther-processed to such, since their polymer structure and thepolydispersity can be controlled very effectively and hence thetechnical adhesive properties can be tailored to the intended end use,and it is nevertheless possible to provide virtually colorless polymersystems. Since the decolorizing reaction of the invention causes nosubstantial alteration to the structure and the composition of thepolymer, the properties of the adhesive are unaffected.

For use in particular as PSAs, the base polymer component (or thealready partly blended PSA) may be admixed with customary additives thatare useful for obtaining and/or improving the pressure-sensitiveadhesion properties, or with additives useful for obtaining and/orimproving other properties.

It is possible in particular, accordingly, to admix resins, moreparticularly tackifying resins. Tackifying resins which can be used are,for example, the tackifier resins that are known and are described inthe literature. In general it is possible to use all resins that arecompatible (soluble) with the corresponding adhesive, reference beingmade more particularly to all aliphatic, aromatic, and alkyl-aromatichydrocarbon resins, hydrocarbon resins based on pure monomers,hydrogenated hydrocarbon resins, functional hydrocarbon resins, andnatural resins.

Examples include the pinene resins, indene resins, and rosins, theirdisproportionated, hydrogenated, polymerized, and esterified derivativesand salts, the aliphatic and aromatic hydrocarbon resins, terpene resinsand terpene-phenolic resins, and also C5, C9 and other hydrocarbonresins. Combinations of these and further resins may be usedspecifically in order to bring the properties of the resultant adhesiveinto line with requirements.

It is optionally possible, moreover, for plasticizers (plasticizingagents), fillers (e.g., fibers, carbon black, zinc oxide, titaniumdioxide, chalk, solid or hollow glass beads, microbeads of othermaterials, silica, silicates), nucleators, expandants, compoundingagents and/or aging inhibitors, in the form, for example, of primary andsecondary antioxidants or in the form of light stabilizers, to be added.

Compatible crosslinker substances may be added in order to generatecrosslinking. Examples of suitable crosslinkers include metal chelates,polyfunctional isocyanates, polyfunctional epoxides, polyfunctionalaziridines, polyfunctional oxazolines, or polyfunctional carbodiimides.Polyfunctional acrylates as well may be used with advantage ascrosslinkers for actinic irradiation.

In the context of the use of the polymers obtainable in accordance withthe invention as adhesives, more particularly as pressure sensitiveadhesives, it is appropriate for them to be made available in the formof an adhesive tape, applied to one or both sides of a carrier. Theadhesive tapes may have further layers, such as, for instance, furthercarrier layers, functional layers, or the like.

For the anchoring of the PSA on the carrier or on another substrate itmay be an advantage if the composition and/or the substrate is treated,prior to coating, by corona or plasma. Examples of apparatus suitablefor atmospheric plasma treatment include those from Plasmatreat.

For processing and for the anchoring of the layer of (pressuresensitive) adhesive with further possible layers, such as carriers, forexample, such as a film based on polyester, polyamide, polymethacrylate,PVC, etc., for example, or with a viscoelastic foamed or unfoamedcarrier based on polyacrylate or polyurethane, it may further be ofadvantage for chemical anchoring to take place, via a primer, forexample.

The internal strength (cohesion) of the PSA is preferably boosted bycrosslinking. For PSA use, particular preference is given to employingthose polyacrylate-based compositions which are coordinatively orcovalently crosslinkable, in order to ensure that the adhesive possessesa constant profile of properties. For crosslinkings of this kind, theprior addition of suitable crosslinkers is advantageous, particularlythose of the kind listed earlier on above.

For transport, storage, or diecutting, the single-sided or double-sidedadhesive tape is preferably provided on at least one side with a liner,that is, for example, with a silicone-coated film or silicone paper.

A further advantageous embodiment of the invention is the use of a layerof a carrier-free (pressure sensitive) adhesive in the form of aself-adhesive (pressure sensitive) adhesive tape, in other words as whatis called an adhesive transfer tape. A carrier-free adhesive is anadhesive which has no permanent carrier. Instead, in a preferredconfiguration, the self-adhesive composition is applied merely to atemporary carrier, this being material which serves only temporarily forthe support and easier application of the self-adhesive composition.Such temporary carriers are also referred to as liners, and mayadvantageously exhibit a release effect, by means of suitable surfacecoatings, for instance. For the use of the layer of (pressure sensitive)adhesive for bonding to a substrate surface, the liner is then removed,and the liner therefore does not constitute a productive component.

An (adhesive or nonadhesive) polymer layer of the invention of this kindmay be produced from solution and also from the melt. For the lattercase, suitable production procedures include both batch processes andcontinuous processes. Particularly preferred is the continuousmanufacture by means of an extruder with subsequent coating directly ona liner with or without a layer of adhesive.

The polymer layer of the invention can be produced in variousthicknesses, including, in particular, with a layer thickness of atleast 25 μm, preferably of at least 100 μm, more preferably of at least200 μm.

The invention is elucidated in more detail below by a number ofexamples, without the invention being restricted as a result.

EXAMPLES Test Methods

The following test methods were employed in order to evaluate both thetechnical adhesive properties and the general properties of the PSAsprepared.

Gel Permeation Chromatography GPC (Test A)

The average molecular weights M_(n) and M_(w) and the polydispersity Pwere determined using gel permeation chromatography. The eluent employedwas THF with 0.1 vol % of trifluoroacetic acid. Measurement was carriedout at 25° C. The preliminary column used was PSS-SDV, 5μ (5 μm), 103 Å(0.0103 μm) ID 8.0 mm×50 mm. Separation took place using the columnsPSS-SDV, 5μ (5 μm), 103 Å (0.0103 μm) and also 105 Å (0.0105 μm) and 106Å (0.0106 μm) each with ID 8.0 mm×300 mm. The sample concentration was 4g/l, the flow rate 1.0 ml per minute. Measurement was made against PMMAstandards.

180° Bond Strength Test (Test B)

A strip 20 mm wide of an acrylate PSA applied as a layer to a polyestercarrier was applied by the exposed side to steel plates which had beenwashed twice with acetone and once with isopropanol beforehand, or PEplates. The pressure-sensitive adhesive strip was pressed onto thesubstrate twice with a 2 kg weight. Immediately thereafter, the adhesivetape was peeled from the substrate at 300 mm/min and at a 180° angle,and the force required was measured. All measurements were carried outat room temperature. The measurement results are reported in N/cm and asthe arithmetic mean from three measurements.

Shear Strength (Test C)

A strip 13 mm wide of the adhesive tape was applied to the smoothsurface of a steel plate, which was cleaned three times with acetone andonce with isopropanol, such that the area of adhesion is 20 mm·13 mm(length·width), one of the narrow edges of the area of adhesion adjoinsflush with one of the edges of the steel plate and the adhesive tapeprojects unsupported beyond this edge of the steel plate. Subsequently,a contact pressure of 2 kg was used to press the adhesive tape onto thesteel plate four times in the region of the area of adhesion.

The steel plate was then suspended such that said edge of the steelplate points downward and the protruding end of the adhesive tape hangsdownward in an unsupported manner. At room temperature, a 1 kg weightwas secured to the protruding end of the adhesive tape and the processof detachment of the adhesive tape was observed. The holding powersmeasured are the times after which the adhesive tape fell off thecarrier, reported in minutes, and correspond to the arithmetic mean fromthree measurements.

Color Analysis (L*a*b* Color Space Analysis) (Test D)

The L*a*b* color space is a measurement space that contains allperceptible colors. The color space is constructed on the basis of thetheory of opposite colors. One of the most important properties of theL*a*b* color model is its independence from the instrument, meaning thatthe colors are defined independently of the way in which they areproduced and the reproduction technology. The corresponding Germanstandard is DIN 6174: “Colorimetric evaluation of colour coordinates andcolour differences according to the approximately uniform CIELab colourspace”.

The colors were determined with a BYK Gardner spectro-guide colorimeter.The colors were determined according to the CIELab scale with the D/65°illuminant. In this three-dimensional color space, the following axesare defined:

-   -   L*=brightness (0=black, 100=white    -   a*=red−green (−120=green, +120=red)    -   b*=yellow−blue (−120=blue, +120=yellow)

REFERENCE TILE

-   Bianco Luce White Shiny, S. White Line, 1234567359687 type 01,    20×20×0.7 cm.

Production of the Investigation Specimens and Experimental InvestigationPreparation of bis-2,2′-phenylethyl thiocarbonate (1)

The bis-2,2′-phenylethyl thiocarbonate was synthesized starting from2-phenylethyl bromide with carbon disulfide and sodium hydroxide inaccordance with a procedure from Synth. Communications 1988, 18,1531-1536. Yield after distillation: 72%.

Characterization: ¹H NMR (CDCl₃) δ (ppm): 7.20-7.40 (m, 10H), 1.53, 1.59(2×d, 6H), 3.71, 3.81 (2×m, 2H).

Trithiocarbonate-Functionalized Polystyrene (2)

A 500 ml Schlenk vessel was charged with 400 ml of styrene and 3.47 g ofthe trithiocarbonate (1) (11.72 mmol), the vessel was degassed threetimes and then the polymerization was conducted under argon. Forinitiation, the vessel was heated up to 120° C. and polymerization waseffected while stirring for 30 h. For isolation, the batch was cooled toRT, and the polymer was dissolved in 1000 ml of dichloromethane and thenprecipitated in 7.5 l of methanol with vigorous stirring. Theprecipitate was filtered off using a frit and then analyzed by means ofGPC (M_(n)=23 500 g/mol, M_(w)/M_(n)=1.32).

Example 1

A conventional reactor for free-radical polymerizations was charged with32 g of trithiocarbonate-functionalized polystyrene (2), 442 g of2-ethylhexyl acrylate, 4.5 g of acrylic acid and 0.12 g of Vazo 67®(from DuPont). After passing argon through for 20 minutes and degassingtwice, the reactor was heated up to 70° C. and polymerization waseffected for 16 h. For isolation, the batch was cooled to RT, and thePS-P(EHA/AA)-PS block copolymer was diluted to 50% with acetone and thencoated onto a PET carrier 23 μm thick by means of a conventional coatingbar and then dried at 120° C. for 15 minutes. The coatweight was 50g/m². The polymer has a pale yellowish color. GPC analysis: M_(n)=95 500g/mol, M_(w)/M_(n)=2.24. Subsequently, testing was effected by testmethods B and C (see table 1).

Example 2

A reactor conventional for radical polymerizations was charged with 32 gof n-butyl acrylate, 442 g of 2-ethylhexyl acrylate, 4.5 g of acrylicacid, 1.40 g of 4-(benzothioylsulfanyl)-4-cyanopentanoic acid (5.00mmol, Aldrich, CAS No. 201611-92-9), and 0.12 g of Vazo 67® (DuPont).After argon had been passed through the reactor for 20 minutes and thereator had been twice degassed, the reactor was heated to 70° C. withstirring and polymerization was carried out for 16 hours. For isolationthe batch was cooled to room temperature, the polymer was diluted withacetone to 50%, 0.2 wt %, based on the polymer, of aluminumacetylacetonate (Aldrich, CAS No. 13963-57-0) was added, and theresulting compositions were then coated using a conventional bar coateron a 23 μm PET carrier, followed by drying at 120° C. for 15 minutes.The coatweight was 50 g/m². The polymer has a reddish coloration. GPCanalysis: (M_(n)=104 000 g/mol, M_(w)/M_(n)=2.09). Testing took placesubsequently in accordance with test methods B and C (see table 1).

Decolorization of the Adhesive (Examples 1a and 2a)

1.2 equivalents, based on the molar amount of the RAFT CTA usedbeforehand, of freshly distilled cyclopentadiene are added dropwise tothe polymer solutions prepared in examples 1 and 2 at room temperaturewith vigorous stirring. Subsequently, the solutions were stirred for ahalf hour, in the course of which a distinct decrease in thediscoloration was detectable. The solutions were each only slightlyyellowish and were diluted to 50% with acetone. Subsequently, 0.2% byweight, based on the polymer, of aluminum acetylacetonate (Aldrich, CASNo. 13963-57-0) was added to the solution from example 2, and the twosolutions were coated onto a PET carrier 23 μm thick using aconventional coating bar and dried at 120° C. for 15 minutes. Thecoatweight was 50 g/m² in each case.

This was followed by testing by test methods B, C and D (see tables 1and 2).

Results

TABLE 1 Adhesion data Bond strength Bond to steel strength to HoldingExample [N/cm] PE [N/cm] powers [min] 1 3.2 2.1 >10 000 1a 3.2 2.1 >10000 2 4.5 2.5   5400 2a 4.5 2.4   5500

TABLE 2 L*a*b* values Example L* a* b* 1 86.25 −0.41 3.08 1a 92.28 −0.122.36 2 79.68 −1.52 4.40 2a 92.26 −0.06 2.25

The experiments demonstrate that, after the addition of cyclopentadiene,the color was distinctly reduced while maintaining the adhesiveproperties. The discoloration of the polymers using trithiocarbonates asRAFT CTAs is generally less than that when using dithioesters as RAFTCTAs. Even in the case of trithiocarbonates, very good decolorization ofthe polymers can be detected, but the decolorization reaction whendithioesters are used runs far more efficiently.

1. A method for producing acrylate-based polymers, wherein first acontrolled radial polymerization reaction of an initial reaction chargecomprising at least one acrylate-based monomer is carried out in thepresence of at least one chain transfer agent (CTA) having at least onefunctional S—C═X group, where X═S, O or N, the CTA being selected fromthe group consisting of dithioesters, dithiocarbonates,dithiocarbamates, trithiocarbonates, imidodithiocarbonates, andxanthates (“RAFT CTAs”), the majority of the CTA molecules beingincorporated into the polymer chains that form, whereby the polymerchains have the functional group of the incorporated CTA, wherein atleast one chemical compound which has at least two conjugated doublebonds (“conjugated diene”) is contacted with the polymers thus obtained,so that hetero-Diels-Alder reactions are brought about between thedouble bonds C═X of the functional groups S—C═X incorporated into thepolymer chains and the conjugated double bonds of the conjugated diene.2. The method as claimed in claim 1, wherein the chemical compound whichhas at least two conjugated double bonds is added to the polymersobtained.
 3. The method as claimed in claim 1, wherein the polymersobtained are admixed with at least one chemical compound from which itis possible in situ to generate a chemical compound which has at leasttwo conjugated double bonds.
 4. The method as claimed in claim 1,wherein the compounds having the conjugated double bonds in the unitC═C—C═C are not heterosubstituted.
 5. The method as claimed in claim 1,wherein the conjugated dienes are selected from the group consisting of1,3-butadiene, 1-methoxy-1,3-butadiene, 2,4-hexadiene,(2E)-2,4-pentanedienoic acid, sorbic acid and its esters,2,4-hexadiene-1,6-dicarboxylic acid and its esters, 1,3-cyclohexadiene,1-methoxy-1,3-cyclohexadiene, anthracene, substituted and unsubstitutedfurans, substituted and unsubstituted thiophenes, substituted andunsubstituted cyclopentadienes,(E)-1-methoxy-3-trimethylsilyloxy-1,3-butadiene,1-(trimethylsiloxy)-1,3-butadiene, 1-(triethylsiloxy)-1,3-butadiene, and(1E,3Z)-1-methoxy-2-methyl-3-(trimethylsilyloxy)-1,3-pentadiene.
 6. Themethod as claimed in claim 1, wherein the chain transfer agent (CTA) is


7. The method as claimed in claim 1, wherein the chain transfer agent(CTA) used is a dithioester.
 8. The method as claimed in claim 1,wherein the acrylate-based polymers obtained are used as adhesives orfurther-processed to adhesives.
 9. The method as claimed in claim 8,wherein the adhesives are pressure sensitive adhesives. 10.Acrylate-based polymers, having cyclo-1-X-hex-3-ene units in the polymerchains, with X═S, O and/or N.